Add independent draft panes and sidebar lifecycle filters

[brsbl]

Human comments

What was wrong

New thread shared one browser-local composer, so opening or submitting another composer could consume the same unfinished work. The sidebar could not select saved drafts or archived threads.

What changed

• Give each draft its own composer, options, destination, recoverable edits, and split-pane identity.
• Import the existing singleton only after acknowledged storage; preserve newer edits and other drafts when submitting one.
• Add Active/Drafts/Archived sidebar filtering, default Active, plus SDK/CLI draft opening and lifecycle preference controls.
• Keep schedules and plugin submission data saved through the SDK when the app supplies no replacement. Load legacy import and optional lifecycle rows after the app shell.

This is the approved clean-history replacement for #3531. It retains the six accepted P1 fixes for recovery ownership, archive transitions, new-pane focus, project selection, reasoning normalization, and provider readiness. SDK 0.4.103 adds draft navigation using existing authenticated client delivery. Pane layout and unsaved recovery remain local, with legacy layout projection retained.

How you verified

• HOLD: remote CI at 35097a66766c8cf45eaa49e7db1d4b6c54cb690f passes all app/server/package/integration tests, typecheck/lint/build, and Linux/macOS package smoke. The raw startup bundle exceeds the original cap by 14.0 KB; a budget decision remains pending. No budget increase remains in this diff.
• Remote measurements: main 8d35c2776 1671.8 KB raw / 411.3 KB brotli; parent 720af27ce 1674.8 / 411.3; this head 1697.6 / 416.4. Existing caps are 1683.5 / 419.0. Lazy import and optional-row boundaries reduced the initial implementation from 1725.6 / 423.3. Route closure is 2178.0 / 585.1 against 2215.3 / 591.1. No further optimization or budget increase is approved.
• Chrome for Testing 153.0.8010.36, isolated source app: distinct split drafts, modifier New thread with focus, edits and hard reload, close/CLI reopen, acknowledged singleton import, lifecycle selection/reset with SDK/CLI readback, and submit-one/retain-neighbors passed. The observed saved-schedule defect was fixed and its affected retest produced one pending queued message. Final-head startup and lazy Drafts/Archived rows passed on desktop and compact layouts. Expected missing/consumed-draft 404/410 responses were handled; no uncaught application errors were observed.
• Fresh 2× evidence below compares parent 720af27ce81921013712f1b1a3ad870eeaab220d and this head with the same synthetic fixtures, light theme, route /?draft=drf_ui_regression_list, focused pane, and CSS viewports (1440×960 desktop; 390×844 compact). Full-resolution PNGs were inspected.
• The sole review was completed on Add independent draft composers, split navigation, and sidebar lifecycle filters #3531; no additional review was performed for this replacement.

Desktop before

Desktop after

Compact before

Compact after

BB-Thread-ID: thr_8j56at3ii2

AGENT GENERATED

[brsbl]

Closing as superseded at the user's request. The current implementation builds on the shipped Drafts plugin, threads, and queued messages: #3930 (SDK/CLI lifecycle filtering and save actions), #3933 (sidebar filters), #3938 (palette filters), and #3931 (independent New thread split composers). This PR is no longer a merge candidate. Its branch is retained.