Fix Shift+Enter, Option+Delete, Cmd+Delete in task form fields

[bborn]

Summary

• The task form's title and details fields didn't respond to Shift+Enter, Option+Delete, or Cmd+Delete on modern macOS terminals (Ghostty, WezTerm, Kitty, iTerm2 with modifyOtherKeys).
• Those terminals send CSI-u sequences like \x1b[13;2u and \x1b[127;9u, which Bubble Tea v1.3.10 emits as unknownCSISequenceMsg rather than tea.KeyMsg. The form only inspected tea.KeyMsg, so the keys went nowhere.
• Translate the recognized CSI-u and xterm modifyOtherKeys sequences back into canonical tea.KeyMsg values at the top of FormModel.Update. The existing textinput/textarea bindings then do the work — no new edit logic.

Mappings applied:

• Shift+Enter → newline in the body field
• Option+Delete / Option+Backspace → delete previous word
• Cmd+Delete / Cmd+Backspace → delete to start of line (matches macOS native behavior)
• Cmd+Left / Cmd+Right → jump to line start / line end

Unknown CSI sequences pass through unchanged, so non-targeted behavior is identical to before.

Test plan

• go test ./... — all packages pass
• go test ./internal/ui/ -count=1 -race — clean under the race detector
• go vet ./... — clean
• New unit tests in internal/ui/key_csi_test.go exercise real unknownCSISequenceMsg values produced by a live Bubble Tea program and confirm each sequence maps to the right tea.KeyMsg (and that unknown / non-CSI messages pass through untouched).
• New integration tests in internal/ui/form_modern_keys_test.go drive FormModel.Update and verify body/title edits for Shift+Enter, Option+Delete, and Cmd+Delete in both kitty and modifyOtherKeys forms — and confirm the legacy tea.KeyMsg paths still work.
• Manual: open the new-task form in Ghostty/iTerm2 (modifyOtherKeys), type into details, press Shift+Enter for a newline, Option+Delete to remove a word, Cmd+Delete to clear the line.

Notes / limitations

• Default macOS Terminal.app does not send these sequences unless "Use Option as Meta key" is enabled and/or modifyOtherKeys is requested. This PR doesn't enable the kitty keyboard protocol from our side — it just consumes the sequences if the terminal already sends them.

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