fix(web): stabilize mobile sidebar gestures and scrolling

[muggle-stack]

Mobile sidebar swipes could stall or snap during release and reversal, especially while a conversation streamed. On iOS, immediately flicking the session list after opening the sidebar could also stop its momentum midway. This change reduces rendering work during navigation and keeps the native scroll layer hierarchy stable across opening and animation cleanup.

• Isolate sidebar state from the conversation owner and memoize unchanged Markdown while retaining current callbacks and streaming updates.
• Reserve horizontal navigation before browser gesture arbitration, preserve native vertical/nested scrolling, and match release motion to finger velocity. Keep fade, blur, and rounded corners proportional to drag progress.
• Keep mobile page layers stable at rest and fade a separate sidebar veil. Leave opening animations alone until horizontal intent is established, cancel abandoned long presses, and avoid redundant document scroll resets.
• Portal image previews and paste editors to the document body so viewport overlays remain correctly positioned with the persistent transformed page. The initial uncompressed bundle cap increases by 1 KiB to accommodate less than 0.1 KiB of added portal code; compressed-size and request limits are unchanged.

Validation on the exact source snapshot committed here:

• Full local gate: pytest 5342 passed, 4 expected skips; Ruff 0.15.13; Node 24 Web build, reliability tests and lint; shell syntax, ShellCheck, and diff checks all passed.
• Chromium/WebKit sidebar diagnostics: 68 passed, 6 expected platform skips. Image, Mermaid, and paste overlay regressions: 14 passed.
• Deployed snapshot passed runtime/build identity, connectivity, and service-stability checks. The maintainer's iPhone PWA retest confirms that immediate list scrolling no longer loses momentum and sidebar opening/closing is substantially smoother.

Known limitation: a small first-drag jitter remains on the maintainer's iPhone. This PR improves that behavior but does not claim to eliminate it. Desktop WebKit diagnostics do not replace physical iPhone frame-timing measurements.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-07T07:54:17.051154Z	3662827	Manual request

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[muggle-stack]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: 3662827392

ℹ️ About Codex in GitHub

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