feat: copy text from panels [2] and [3] by selecting it with the mouse

[stanlyzoolo]

Text in [2] and [3] was on screen but unreachable: the terminal's own selection is gone under the alt screen with mouse reporting on, so a version string, a command from a README fence or a release note could only be retyped by hand.

Drag with the left button in the brief card or the docs panel to select, release to copy the plain text to the system clipboard. The status bar reports copied N characters, or copy failed: <err> when the clipboard write fails.

How it works

• handleSelectableMouse (render.go) is the shared driver for both panels: Press anchors, Motion extends, Release copies. A drag belongs to the panel it was anchored in, so a release over [1] or outside the viewport still finalizes instead of stranding selActive with the highlight frozen on screen.
• Coordinates are content coordinates (selPos{line, col}), not screen ones — YOffset is added when mapping a row back to a line and never stored, so the selection survives scrolling. paintSelection auto-scrolls at the window edges so a drag can extend past what is visible.
• beginSelection snapshots the panel's rendered lines (styled + ANSI-stripped) at press time, so a motion event repaints without re-rendering the card or the README.
• The highlight is ansi.Cut + reverse video, applied per whole line — the codebase's strip-then-repaint rule, so a cut can never land inside an escape sequence.
• Copy runs off the Update thread (copyCmd): clipboard.WriteAll shells out to pbcopy/xclip/wl-clipboard/PowerShell and can block.

Behavior changes

• A card link now opens on Release, not on Press — the press anchors a drag, and a drag starting on the repo line must copy rather than open the browser. Only a motion-less release opens the link.
• Any keystroke abandons an in-flight drag: a release is not always delivered (the button can be let go over another window), and a key must never act on content still wearing the highlight.
• Selection is gated exactly like every other click: modeNormal only, nothing while an overlay is up.
• github.com/atotto/clipboard moves from indirect to a direct dependency.

Tests

selection_test.go covers copy on release, a drag starting on a link not opening it, an empty click copying nothing, a release outside the panel finalizing, and the overlay/non-normal gate. textutil_test.go covers highlightLine, selectedText and highlightSelection as pure functions; commands_test.go drives the copyDoneMsg handler through the writeClipboard seam.

Preflight green: build, vet, test -race, golangci-lint.