internal/ui, internal/gutil, internal/tsutil: drop unused fields and helpers

[DeedleFake]

Opened by Coder (Grok Bot), not typed by DeedleFake.

Drop unused GTK fields and helpers. Deslop only; no other cleanup.

1. internal/ui/peerpage.go — Drop leftover FillFromUI struct fields that are not in peerpage.ui and are never read: SelfPage/netcheck widgets (UDPRow/UDP, IPv4*, IPv6*, UPnP*, PMP*, PCP*, HairPinningRow/HairPinning, PreferredDERPRow/PreferredDERP, DERPLatencies) and old send-file widgets (SendFileBurron, SendDirButton). Netcheck widgets remain on selfpage.ui. Advertised Routes is untouched.

2. internal/gutil/gutil.go — Delete unused Classy and SetCSSClass (zero callers). AddCSSClass continues to be used directly in dialogs.go / app.go.

3. internal/tsutil/tsutil.go — Delete unused CompareWaitingFiles. Not wired into waitingFileSorter; the sorter in internal/ui/ui.go already inlines the same name-then-size cmp. No tests existed for it.

4. internal/ui/dialogs.go — Remove Select.Multiple and the multi-select GTK #552 workaround. The only caller (Select[tailcfg.NodeView] in onAppOpen) never set Multiple. Select is now single-select only. No tests mentioned Multiple.

5. internal/tsutil/poller.go — Replace mk.Chan/mk.Map with make() for poll/getIPN/nextIPN/interval channels and the Peers map. Drop deedles.dev/mk from go.mod/go.sum. Confirmed no other file imported it.

Nothing listed was skipped; all five items were still present and unused.