feat: show unread counts

[mavonx]

What?

Shows unread email counts next to folder names in the sidebar.

Why?

Folder list showed folder names only, making it impossible to see which folders have unread mail without opening each one.
This improves navigation efficiency.

Closes #645

[floatpanebot]

Hi @mavonx! Please fix the following issues with your PR:

• Title: Is too long (45 characters). The PR title must be strictly under 40 characters.

Formatting issues have been resolved. Thank you!

[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    2.092µ ±  88%   2.912µ ± 23%        ~ (p=0.180 n=6)
ParseSearchQuery_Complex-4   5.631µ ± 108%   7.308µ ± 60%        ~ (p=0.485 n=6)
TokenizeSearchQuery-4        4.991µ ±  27%   4.546µ ± 33%        ~ (p=0.288 n=6)
geomean                      3.888µ          4.590µ        +18.05%

                           │  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        154.4µ ± 14%   159.2µ ± 21%       ~ (p=0.937 n=6)
SearchOverlayView-4   164.9µ ±  6%   176.9µ ± 14%       ~ (p=0.310 n=6)
InboxConstruction-4   578.3µ ± 20%   627.6µ ± 34%       ~ (p=0.589 n=6)
geomean               245.1µ         260.5µ        +6.28%

                    │    old.txt    │               new.txt               │
                    │     B/op      │     B/op       vs base              │
LogPanelView-4        44.67Ki ± 51%   44.67Ki ± 51%       ~ (p=0.636 n=6)
SearchOverlayView-4   44.66Ki ± 26%   44.66Ki ± 26%       ~ (p=0.515 n=6)
InboxConstruction-4   873.9Ki ±  0%   874.0Ki ±  0%       ~ (p=0.591 n=6)
geomean               120.4Ki         120.4Ki        +0.00%

                    │   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%    925.0 ± 0%       ~ (p=1.000 n=6)
InboxConstruction-4   3.475k ± 0%   3.476k ± 0%       ~ (p=0.545 n=6)
geomean               1.319k        1.319k       +0.00%

_{auto-generated by benchmarks.yml}

[andrinoff]

lgtm

[andrinoff]

lgtm