fix(tui): settings focus nav

[FromSi]

What?

Improves Settings pane focus navigation in the TUI.

• Adds left navigation back from the content pane to the settings menu.
• Allows left to leave Settings from the menu, like esc.
• Hides right-pane cursor/highlight when the content pane is not focused.
• Keeps bold styling only on the currently focused pane.
• Updates English Settings help text.
• Adds tests for horizontal pane focus and App Encryption input behavior.

Why?

Closes #1231

Fixes confusing Settings focus behavior where both panes could appear selected at the same time.

This makes keyboard navigation clearer: > and bold styling now indicate the active focus, while left provides a consistent way to move back or exit.

[floatpanebot]

Benchmark report — no significant change

Metrics worse: 0 · better: 0 (threshold: ±3%).

benchstat output

goos: linux
goarch: amd64
pkg: github.com/floatpane/matcha/backend
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
                           │    old.txt    │               new.txt                │
                           │    sec/op     │    sec/op      vs base               │
ParseSearchQuery_Simple-4    3.382µ ± 237%   3.596µ ± 208%        ~ (p=0.589 n=6)
ParseSearchQuery_Complex-4   7.008µ ±  58%   6.171µ ±  28%        ~ (p=0.394 n=6)
TokenizeSearchQuery-4        5.876µ ±  65%   4.257µ ± 218%        ~ (p=0.699 n=6)
geomean                      5.183µ          4.554µ         -12.14%

                           │  old.txt   │              new.txt               │
                           │    B/op    │    B/op     vs base                │
ParseSearchQuery_Simple-4    26.00 ± 0%   26.00 ± 0%       ~ (p=1.000 n=6) ¹
ParseSearchQuery_Complex-4   762.0 ± 0%   762.0 ± 0%       ~ (p=1.000 n=6) ¹
TokenizeSearchQuery-4        176.0 ± 0%   176.0 ± 0%       ~ (p=1.000 n=6) ¹
geomean                      151.6        151.6       +0.00%
¹ all samples are equal

                           │  old.txt   │              new.txt               │
                           │ allocs/op  │ allocs/op   vs base                │
ParseSearchQuery_Simple-4    2.000 ± 0%   2.000 ± 0%       ~ (p=1.000 n=6) ¹
ParseSearchQuery_Complex-4   23.00 ± 0%   23.00 ± 0%       ~ (p=1.000 n=6) ¹
TokenizeSearchQuery-4        9.000 ± 0%   9.000 ± 0%       ~ (p=1.000 n=6) ¹
geomean                      7.453        7.453       +0.00%
¹ all samples are equal

pkg: github.com/floatpane/matcha/tui
                    │   old.txt    │               new.txt                │
                    │    sec/op    │    sec/op      vs base               │
LogPanelView-4        154.1µ ±  5%    158.5µ ±  6%        ~ (p=0.240 n=6)
SearchOverlayView-4   167.5µ ± 11%    167.5µ ± 15%        ~ (p=0.699 n=6)
InboxConstruction-4   890.5µ ±  7%   1001.9µ ±  8%  +12.51% (p=0.002 n=6)
geomean               284.3µ          298.5µ         +5.01%

                    │    old.txt    │               new.txt               │
                    │     B/op      │     B/op       vs base              │
LogPanelView-4        44.67Ki ± 51%   44.67Ki ± 51%       ~ (p=1.000 n=6)
SearchOverlayView-4   44.66Ki ± 26%   56.14Ki ±  0%       ~ (p=0.121 n=6)
InboxConstruction-4   874.3Ki ±  0%   874.2Ki ±  0%  -0.01% (p=0.041 n=6)
geomean               120.4Ki         129.9Ki        +7.92%

                    │   old.txt   │              new.txt              │
                    │  allocs/op  │  allocs/op   vs base              │
LogPanelView-4         714.0 ± 0%    714.0 ± 0%       ~ (p=1.000 n=6)
SearchOverlayView-4    925.0 ± 0%    926.0 ± 0%       ~ (p=0.121 n=6)
InboxConstruction-4   3.479k ± 0%   3.478k ± 0%  -0.04% (p=0.022 n=6)
geomean               1.320k        1.320k       +0.02%

_{auto-generated by benchmarks.yml}

[andrinoff]

great!

[FromSi]

@andriy-chernov need i18n, thx

[andrinoff]

@andriy-chernov need i18n, thx

he is handling some things today, i'll do it