Build: state and district populations, and indexing in a pop-up

[PavelMakarchuk]

Summary

Reports from Build ran nationwide only. This PR lets a reform run on any US state or congressional district, which are the subnational regions that the API supports. It touches v3 (flagship) code only.

• Population menu in Build. It offers Nationwide, the 51 states and the 436 congressional districts, and you can search them by name or code ("ca", "calif", "CA-01"). Districts join the list once something is typed, so the list stays short. "A household" stays marked Soon; household runs come in a follow-up PR.
• The run uses the region. The draft stores it (population.region, for example state/de or congressional_district/CA-12), and runFlagshipReport creates both simulations on it. The report's calculation then runs with that region.
• Report cover. The line under the title reads the population and the year, for example "Delaware · 2026" or "Nationwide · 2026". It no longer reads "Hand-built". "Edit the reform" brings the region back to Build.
• Recompute keeps the population. The adjust panel now recomputes on the report's region and year. Before, it ran nationwide and in the current year, even for a state report or a 2027 bill.
• Bills from the Reforms page. "Run impact report" for a state bill now runs on its state, as the bill's own report does.
• Headline revenue. The headline leads with state revenue only when a state reform runs on part of the country (leadsWithStateRevenue). A federal reform on one state leads with its full budget effect. A $100 CTC increase run on Delaware now shows "$12.8m annual government cost", where it showed "$0.0 state revenue change".

Build: edit modes, indexing in a pop-up, aligned columns

• Modes: New value | By year | Custom dates | Indexed. New value is the default; Indexed is last.
  • New value is a single box with current law next to it. An Applies from menu sets the year it starts; the years before it stay at current law. A closed row then reads, for example, "$1,000" and "from 2028".
  • Indexed (money amounts only) shows the value, what it grows to in the later years, and "Edit indexing". Choosing it indexes the amount at once: at current law's rate when current law indexes the amount, otherwise with the first index offered.
  • A new value holds flat until Indexed is chosen, also on an amount that current law indexes. Only a provision that already indexes opens in Indexed.
  • Back in New value, an amount never edited returns to current law, and an edited one holds from its year.
• Pop-up. Edit indexing opens a pop-up with the index, the start year, the base year, and the rounding mode and step. It shows the live formula and has Cancel / Apply. "None" is not offered, because New value covers it.
• Indexing changes the values. Indexing applies also to an amount still at current law. A flat $100 indexed to CPI-U shows $102 … $122 by 2035.
• Aligned grid. The year columns share the width evenly, and each heading starts at its values' "$". The current-law row and the later years lay out values as the input box does (ValueDisplay). A phone scrolls the grid instead of squeezing it.
• Wording. "No values differ from current law yet. Adjust at least one value to run the report." "Show past values" now reads "Show historic values".
• The Population menu names a state ("Arkansas") while metadata loads, not "state/ar".

Report side panel

• Population and year. The panel has the same Population menu, and a recompute runs on the chosen region and year.
• Rename. The reform's name can change in the panel; a rename alone is a change to recompute.
• "Baseline: {value}" replaces "was" beside each editor. Booleans edit as a true/false switch.
• Edit in Build takes the adjusted reform (name, year, region) back to Build to add more provisions.

Saved reforms (Reforms page)

• The source labels ("Hand-built", "From a question", …) no longer show on the cards or the detail.
• Dates are kept. The detail read only a reform's first value against today's law ("$500 → 1000" for a value from 2028). Its report ran that value from the current year, and "Save changes" kept only the first period of a schedule. Now:
  • each row says when its value applies ("from 2028", "3 periods from 2026"), against current law in that year
  • a one-period value edits in place and keeps its dates on save; a schedule reads as its summary and saves whole (it changes in Build)
  • "View full impact report" runs every dated value, in the reform's first year of change

Progress bar

• A society-wide run shows a bar with the elapsed time and a percentage (RunProgress). The API reports no stages for a single-year run, so the bar follows the typical run time (US 6 min, UK 3 min), and stops at 95% until the results arrive.

Test plan

• tsc, ESLint and Prettier are clean.
• Full Vitest: 4,220 passed. New tests cover:
  • the picker (choosing a state, searching for a district)
  • the run sending the region to both simulations
  • recompute keeping the region and year
  • the state revenue rule
  • the indexing pop-up: Apply on an unedited amount, and Cancel
  • the mode order, New value as the default, and "Applies from" moving the start year
  • the side panel: population, rename, Edit in Build
  • the report cover: population and year, no "Hand-built"
  • saved reforms: a later start shown, kept on save and in the run; a schedule saved whole
• On localhost:
  • A flat amount indexed to CPI-U: the later years show the indexed values, and the "$" and the digits line up in every column.
  • A new value with "Applies from" 2028 on a 2026 report: 2026–2027 stay at current law, and the closed row reads "$1,000" and "from 2028".
  • Typing "CA-12" finds California's 12th district, and choosing it sets the draft's region.
  • A Delaware run calls /us/economy/…?region=state/de and finishes. The headline reads "$12.8m Annual government cost".

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