fix(imap): STARTTLS for non-993 ports

[kawacukennedy]

What?

Use STARTTLS for all non-993 IMAP ports, matching the SMTP path logic.

Why?

Fixes #1688

Matcha previously only used STARTTLS on ports 1143 and 143, falling back to implicit TLS on every other port. This broke local IMAP bridges (e.g. Proton Mail Bridge) that serve IMAP on non-standard ports with STARTTLS. Port 993 uses implicit TLS; all other ports now correctly use STARTTLS.

[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: AMD EPYC 7763 64-Core Processor                
                           │   old.txt    │               new.txt                │
                           │    sec/op    │    sec/op      vs base               │
ParseSearchQuery_Simple-4    2.585µ ± 17%   2.956µ ± 128%        ~ (p=0.394 n=6)
ParseSearchQuery_Complex-4   7.688µ ± 19%   7.674µ ±  34%        ~ (p=0.699 n=6)
TokenizeSearchQuery-4        3.674µ ± 26%   4.793µ ± 115%        ~ (p=0.169 n=6)
geomean                      4.179µ         4.772µ         +14.19%

                           │  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         159.4µ ±  8%   171.4µ ± 19%       ~ (p=0.132 n=6)
SearchOverlayView-4    176.1µ ±  5%   174.2µ ± 13%       ~ (p=0.937 n=6)
InboxConstruction-4   1004.2µ ± 19%   969.5µ ±  7%       ~ (p=0.485 n=6)
geomean                304.3µ         307.0µ        +0.89%

                    │    old.txt    │               new.txt               │
                    │     B/op      │     B/op      vs base               │
LogPanelView-4        33.23Ki ± 34%   44.67Ki ± 0%        ~ (p=0.121 n=6)
SearchOverlayView-4   33.17Ki ± 69%   56.15Ki ± 0%        ~ (p=0.091 n=6)
InboxConstruction-4   874.2Ki ±  0%   874.3Ki ± 0%        ~ (p=0.485 n=6)
geomean               98.77Ki         129.9Ki       +31.54%

                    │   old.txt   │              new.txt              │
                    │  allocs/op  │  allocs/op   vs base              │
LogPanelView-4         713.0 ± 0%    714.0 ± 0%       ~ (p=0.182 n=6)
SearchOverlayView-4    923.5 ± 0%    926.0 ± 0%  +0.27% (p=0.045 n=6)
InboxConstruction-4   3.477k ± 0%   3.478k ± 0%       ~ (p=0.734 n=6)
geomean               1.318k        1.320k       +0.15%

_{auto-generated by benchmarks.yml}