codeSplitters UI polish, part 2: leftovers from #678 #710

#678 closed when #689 fixed the ⌘K palette, but these items from the owner's click-through (#609) weren't done yet:

• Following looks like an error. A pressed Follow button (`.btn.follow[aria-pressed="true"]`, `apps/codesplitters/src/client.html:60`) gets an orange border, text and tint, which reads as a warning. Keep the button neutral and turn only the eye icon orange.
• VS Code-style multi-select for line actions. Today it's click a line number, shift-click for a range. Add ⌘-click to add or remove single lines and drag down the gutter to select a run, for Make private, Hold, Share lines, Stamp, Cut and Collect. Make the selected state clearly different from a line open for editing.
• Private lines aren't visibly private. After Make private the line looks like any other. Give it a clear 🔒 in the gutter.
• Upstream's branches list runs on forever (mirror box, around `client.html:1991`). Each branch gets a name line plus a full-size Open here button, so RustyBuns' hundreds of branches make a huge sidebar. Past ~8: a scrolling box with a max height and a filter, one compact row per branch (name + a small ⤓ icon button, "Open here" as its tooltip), newest first.

Owner's taste: obvious icons over words that wrap; labels stay on one line. Related: #701 (branch switcher and branch names).

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