Fix AppSnap, Linux startup, and chat controls

[ojowwalker77]

What changed

• make AppSnap's physical modifier chord configurable across Settings, typed IPC, the desktop manager, and the native helper
• surface permission/runtime failures outside Settings and keep toast descriptions visible
• skip TeaCode's own frontmost window and capture the next eligible visible app window
• prevent Linux startup from quitting when transient storage-migration windows close before the real window is created
• allow the composer to collapse during active turns and pending prompts
• prevent split-pane header controls from shifting under the pointer when an unfocused pane is clicked
• default run checklists to the compact composer placement

Why

AppSnap could fail with an opaque toast, could not change its hardcoded shortcut, and rejected captures initiated while TeaCode was frontmost. Linux could exit during startup because hidden migration windows triggered window-all-closed. Chat controls also had two interaction races: the composer was locked open during active work, and focusing a split pane during pointer capture could move its close button before click completion.

Validation

• bun fmt
• bun lint (0 errors; repository warnings remain)
• bun typecheck (8/8 packages)
• desktop: 23 files, 182 tests passed
• AppSnap helper compiled, signed, and reused successfully
• desktop packaging config: 6 tests passed
• web: changed-area tests passed; full suite passed 1,883/1,884 tests, with only ChatMarkdown > uses the theme foreground token for markdown text hitting its existing 5-second timeout under full-suite load; that file passes alone, 14/14

Scope note

The Control Room implementation is intentionally excluded from this PR and preserved locally in the named Git stash Control Room implementation.