Memoize sidebar timer name lookups

[aldoborrero]

Summary

• Memoize the sidebar timer's project/task name lookups using the same resource-driven pattern as the timesheet.
• Borrow the cached maps when building the running label, keeping the one-second display tick independent of rebuilding all names.
• Add a deterministic component regression covering repeated renders, renaming, resource errors, recovery, and unchanged timer invalidation state.

Validation

• The regression includes the production widget source with controlled resource responses and a counted name-copy boundary. On the original component, 100 extra renders produce 101 copies of each fixture name instead of 1; with this change, the counts stay at 1 until the relevant resource changes.
• cargo test -p horae --features server --test timer_widget --locked --offline passes.
• Server all-target Clippy with -D warnings, WASM compilation, Rust formatting, and git diff --check pass.
• The existing three WASM warnings remain unchanged and are not suppressed by this PR.
• The existing CI test command discovers the new integration-test target automatically.

No SQL, schema, dependencies, timer clock logic, start/stop actions, or database contents changed. The copy counts are a regression assertion, not a wall-clock performance estimate. No live-browser test or full database suite was run locally for this scoped change; required CI runs the full suite before merge.