refactor(input,ui): track input mode per tab instead of globally

[roramirez]

Summary

Moves the InputMode field from the global AppState onto per-tab TabState, so each tab keeps its own Insert/Normal/Visual/Search mode. Switching tabs now restores that tab's own mode instead of sharing one mode across the whole window — matching how zoomed and passthrough are already handled (per-tab, session-only, never persisted).

Changes

• mode field moved from AppState to TabState; initialized to Insert at all three TabState construction sites (new_tab, session restore, add_empty_tab).
• Access routed through panic-free mode() -> &InputMode and set_mode(m) accessors that delegate to the active tab, with a static DEFAULT_MODE = Insert fallback for the empty-tabs case.
• do_set_mode refactored to compute the resolved mode into a local before taking &mut self, avoiding a double borrow.
• Added on_active_tab_changed() to clear/recompute the global search_matches (which describe only the active tab's grid) on next_tab/prev_tab/do_go_to_tab.
• ~70 read/write call sites migrated across app_event, app_state, mouse_ops, input_ops, render_ops, and views.

Invariant

Mode stays session-only and is never serialized — src/session/mod.rs has zero mode references.

How to test

1. cargo run and open a second tab (Ctrl+T).
2. In tab 1, enter NORMAL mode (Ctrl+[ / your normal-mode binding) — the mode badge shows NORMAL.
3. Switch to tab 2 (Ctrl+PageDown / Ctrl+Tab) — the badge shows INSERT (tab 2 keeps its own mode).
4. Switch back to tab 1 — it is still in NORMAL. The mode did not leak across tabs.
5. In tab 1, enter VISUAL mode and start a selection, switch away and back — the tab returns to its own VISUAL state; tab 2 is unaffected.
6. Open scrollback search (/) in one tab and switch tabs — no stale match highlight appears on the other tab (search_matches are cleared/recomputed on switch).

Automated coverage: cargo test — new per-tab isolation tests assert that a new tab starts in Insert, mode is tracked per tab across switches, Visual does not leak between tabs, and search_matches are cleared on tab switch.

Checks

• Full suite green: 1203 + 256 passing.
• cargo fmt --check clean; cargo clippy --locked -- -D warnings clean.