refactor(flterm): separate controller and view responsibilities

[elias8]

Separate terminal-session behavior from Flutter view state so TerminalController owns libghostty state, geometry, selection, and backend callbacks while TerminalView owns focus, text input, scrolling, gestures, and rendering lifecycle.

• Keep TerminalController as an abstract contract for application-level test doubles; move focus and soft-keyboard control to TerminalView, make callbacks setter-only, stop exporting KeyboardState and TerminalScrollPosition, and reject concurrent view attachments.
• Route measured geometry through one controller-owned transaction, emit in-band resize output before backend onResize, and keep terminal, mouse, and selection geometry synchronized.
• Harden input and lifecycle handling across AltGr, IME newline/deletion pairing, lock modifiers, mouse and stylus button transitions, gesture ownership, controller swaps, and font-data changes, with focused coverage for each behavior.

Supersedes #119