feat(terminal): persist font size per terminal tab

[atacolak]

summary

• make terminal font-size shortcuts operate on the focused terminal tab only
• persist per-terminal-tab font-size overrides in session state
• keep app-level font_size as the default for terminals without an override
• preserve backwards compatibility with old session json that lacks font_size

why

currently ctrl++, ctrl+-, and reset behave globally: changing one terminal changes every live terminal surface. in a split workspace this is surprising because each terminal tab is visually independent and often has different content density needs.

this patch stores font-size override state on each terminal tab and applies ghostty font-size actions only to the focused terminal surface.

testing

• cargo fmt --check
• cargo check -p limux-host-linux
• cargo test -p limux-host-linux
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• ./scripts/check.sh
• manual gui check: two terminal panes can be set to different font sizes without changing each other

[atacolak]