feat(app): polish mobile session navigation and drawers

[Brendonovich]

Summary

• Session view switcher: narrow screens show Session, Changes and a More... tab. More... opens one bottom drawer listing Files, Terminal, Usage and Session details. Its options use the same card and row styling as Session details, with the current view highlighted and checked. Files and Terminal now use the menu mobile kind; MobileView gains an optional icon. The switcher moves below the composer when the titlebar is set to the bottom.
• Nested drawers: DrawerContext gains open({ title, content, trigger }), which lets a drawer panel swap in nested content with a back control. Session details uses it to open the MCP, Plugins, Skills and LSP lists inside the same drawer instead of a popover over it.
• Composer and model picker: on mobile, the model, variant and agent pickers open in the shared bottom drawer, the editor loses focus after submit, and controls that are hidden until hover stay visible on touch devices.
• Shell spacing: panels use a new --shell-inline-inset (1px on mobile, 8px otherwise). With the bottom titlebar, mobile panels drop the extra top inset; with the top titlebar, the tab picker sits 8px from the top edge and 8px above the content, keeping the 16px status-bar clearance from fix(app): keep the phone titlebar clear of the iOS status bar blur #53297 only when there is a top safe area. Ghost-muted button hover only applies on devices that support hover.

Testing

• bun run check
• Playwright: mobile-session, mobile-extensions and the new mobile-extension-drawers, mobile-session-options, mobile-composer-submit-focus, mobile-model-pickers and shell-panel-spacing specs pass.
• review, session-summary and side-panels pass. The mobile-timeline-scroll touch-gesture failures and the narrow-screen palette pick failure also happen locally on unmodified v2.