feat(merge_requests): save private drafts on verified diff selections

[sjungwon03]

Summary

The MR changes screen can save a private draft on a selected text line or forward multiline range. The separate dialog shows the original version, paths and coordinates and leaves unsent public discussion text intact. It refreshes MR identity and the authoritative latest diff before one existing positioned-draft request; changed version/SHA/selection or selected text refuses dispatch.

Account/client/repository/snapshot/view replacements isolate obsolete commands and private input. Uncertain writes retain the shared inspection gate across refresh and reopen; complete visible inspection and explicit new consent are required for another manual save. A last-page session check prevents queued client replacement from adopting old private rows or clearing uncertainty.

Closes #616

Validation

• dart format .: 936 files, no changes outstanding
• flutter analyze: no issues
• Full tests: 6,831 (app 3,934; design system 127; API 2,589; models 176; secure storage 5)
• 113 new behavior tests; missing position forwarding/fresh validation and missing UI reproduced first. Snapshot refresh during loading, origin disposal and final-page recovery races reproduced before fixes
• Five locales, 390/800/1200 widths, both themes, large text and keyboard constraints; twelve synthetic widget captures rendered and inspected
• No model changes; localization delegates regenerated

Checklist

• One linked issue, feature branch into dev, Conventional Commit and DCO
• Test-first behavior and updated documentation
• Shared responsive UI; light/dark and narrow/wide checked
• No dependencies, disk persistence or telemetry added
• Dummy credentials, paths and accounts only; English source/docs/review with localized translations

Screenshots

All twelve synthetic captures

Mobile composition, light	Desktop inspection, dark

Notes for reviewers

Uses the existing Draft Notes API position contract and strict create acknowledgement, without public posting, publication, approval, merge, redirect, auth replay or automatic retry. Immutable selected text and a snapshot lifecycle token guard fresh reads and additional refreshes while already loading.

The fresh check is a preflight, not an atomic conditional server write. Recovery is in-memory; identical text/positions cannot identify an uncertain create attempt. No live-instance/device or exactly-once claim. Image/file/commit/reply drafts, single-note publication and reviewer-state controls remain separate; MW-07 stays in progress.

[sjungwon03-ai]

Review of commit 3eecb56: Reviewed the diff, five-locale delegates, behavior documentation and twelve synthetic captures. The controller refreshes MR identity and the authoritative diff, verifies the immutable original selection and reserves discussion commands before the single existing private-create call. Version, selected text, snapshot lifecycle and session changes prevent stale dispatch; dispatched uncertainty survives same-account refresh. The separate modal preserves public input, discards obsolete private state, and requires complete visible inspection plus renewed consent for manual retry. Final inspection adoption now rechecks the session after page validation.

113 new regressions cover sides/ranges, fresh changes, shared reservation, unconfirmed acknowledgements, cancellation/late failures, inspection, localization and responsive layouts. All 6,831 tests, clean analysis/format and four required exact-head CI jobs passed. No blocking findings. Fresh checks remain preflight and recovery is in-memory; live-instance/device validation is not claimed.