Support custom key combinations in mobile terminal shortcuts

[powerfooI]

Addresses #308: terminal applications and user-defined keybindings can require combinations missing from the built-in mobile shortcut presets.

Users can choose Preset or Custom keyboard for each panel or side slot. The US QWERTY picker supports multiple Ctrl, Alt, and Shift modifiers with one main key, shows the selected combination, and updates number/symbol keycaps with Shift. Symbol keys are directly visible in two keyboard rows; navigation keys and F1-F12 appear in separate groups under More keys, with F1-F12 in three consecutive rows. Existing presets, saved layouts, labels, and restore defaults remain available. Custom buttons work in the floating panel, side controls, and composer and send the configured combination without appending Enter.

Collapse terminal encoding guidance under How shortcuts work and shorten the dialog introduction. Prevent mobile slot sections from shrinking and keep the zoomed page stationary while the dialog scrolls. Load this dialog on demand using the existing settings-dialog pattern. Configuration drawer buttons retain left-aligned titles and descriptions. Remove the entire mobile Configuration footer, including Done, the status text, and its separator; the top close button remains available.

Unsupported combinations, such as Ctrl+1, identify the affected slot and key combination, explain the current terminal encoding limitation, and block saving. Character labels preserve case when it affects the sent bytes; compact default labels distinguish keys with multiple modifiers. Cmd/Meta, text macros, and multi-step sequences remain out of scope.

Closes #308.

Validation

• bun run precommit through the pre-commit hook: formatting, lint, type checks, frontend build and existing asset budgets passed; 2365 tests passed, 2 skipped.
• bun run build:web: passed with existing asset budgets.
• Focused shortcut tests passed, including contiguous F1-F12 ordering, invalid-combination messages retaining the affected slot when another slot is selected, picker mode switching, multiple modifiers with one selected key, symbols in the main keyboard, Shift symbol display and saved Alt+Shift+[ bytes, save blocking, panel/side storage, case-sensitive labels, and sent bytes without an added Enter.
• Manual Codex in-app browser on macOS against the real LAN development backend: 393px mobile width, 120% interface scale, short 600px viewport, and wide 1280px viewport; no keyboard overflow or compressed side section, stationary dialog bounds during focus scrolling, collapsed/expanded help and extra keys, directly visible symbol rows, Ctrl+/, shifted symbols and Alt+Shift+[, F1-F12 in a separate three-by-four grid, named Ctrl+1 errors persisting when another slot is selected and clearing on correction, preset menu Escape behavior, and cancellation preserving the original configuration. Interface scale and temporary viewport were restored.
• Manual Edge on macOS against a real backend: add/edit/remove, reload persistence, exact Ctrl+X/Ctrl+Alt+X bytes from all three button locations without extra Enter, and custom-label preservation. Mobile configuration text alignment was verified in the Codex in-app browser. The same browser confirmed that the mobile Configuration footer has no layout footprint, the top close button works, and the desktop footer remains available.

The updated picker has not been verified on physical iOS Safari or Android Chrome devices.