release: Memmy v1.1.5 attachment fixes

[syzsunshine219]

Prepare Memmy v1.1.5 from main (a63ae705245d3eb87d6addacaecca672b75496b6) with the scoped attachment hotfixes. The candidate source is fc4513fb81341b209ff2a91b39079f76e711c595.

• Ignore duplicate attachment drops while preserving unsupported-file and read-error reporting (MEMMY-399).
• Remove WebUI/API multipart attachment count and per-file size limits; use a 256 MiB request body cap and preserve the base64 data URL guard (MEMMY-400/402).
• Synchronize root, Desktop, Agent, and generated backend metadata to 1.1.5. Memory remains 2.1.2.
• Include the documentation, screenshots, and GitHub/Yunxiao synchronization already present in main since v1.1.4; release notes document that baseline.
• Keep the larger candidate on release/v1.1.6. No attachment manifest, IM buffering, or document extraction changes are included in this patch.

Pre-merge validation for this exact SHA:

• Project Harness Full/T3: verificationPassed=true, deliveryReady=true, decision=needs_review; 11 passed, 1 not applicable (local API contract matrix). All lint, affected typechecks/tests, UI assertions, smoke, full build, distribution, and diff checks completed in one serial run.
• Backend: 844 passed. Frontend: 1,544 passed. Agent: 5,134 passed, 3 skipped. Desktop shell: 249 passed, 106 platform skips.
• Desktop interaction/layout assertions: 104 passed. Selected critical smoke plan: 3 passed.
• Release workflow tests: 36 passed, 5 conditional skips. Packaging guards: 32 passed. Version consistency and diff checks passed.
• Regression characterization against old main behavior failed as expected for duplicate-drop alerts and attachment upload limits, then passed in the candidate.

Independent maintainer review is still required. The PR remains Draft for the Harness handoff; local verification is not approval to merge or publish. Signed installers and their installation/upgrade validation, checksums, and release evidence must be prepared from this exact candidate before the release workflow consumes them. No merge to main, tag, installer upload, or release publication has been performed by this task.

[syzsunshine219]

Project Harness review handoff marker. Only reviews submitted strictly after this GitHub timestamp qualify.