Allow customizing the text label on labeled widgets (e.g. "Model: ", "Ctx: ") #600

Many widgets put a hard-coded text label in front of their value when raw value is off: Model: , Ctx: , In: , Session: , Block: , Cost: , S:, and so on. The label can't be changed, only turned off completely with raw value, so a line either spends a lot of width on labels or has no labels at all:

Model: Opus 4.6 | Ctx Used: 42.1% | Session: 23.4% | Cost: $1.20
Opus 4.6 | 42.1% | 23.4% | $1.20

Something in between, like M Opus 4.6 | ctx 42.1% | 5h 23.4% | $1.20, currently needs a Custom Text widget merged in front of each raw-value widget. That's one extra widget per label, and it has to be kept in step with the value widget's colors and merge settings.

Glyph widgets already allow this: Git Branch, the JJ widgets, Git Staged, etc. let you replace or remove their prefix symbol with (g)lyph. Text labels have no equivalent.

Proposal: add a per-widget label override for widgets that render a text label:

• An editor keybind opens a text field with the current label, showing the default (Tab restores it).
• The override replaces the label verbatim, so the user decides the trailing space/colon. An empty label renders the value on its own, while raw value still works as before.
• Widgets whose label depends on their mode (Ctx Used:/Ctx Left:, Block: vs Block in bar mode) use the one override in every mode.
• It only applies while raw value is off, so the keybind can be hidden in raw value mode.
• It's stored in metadata, so no settings schema change or migration is needed.

Glyph-prefixed widgets keep (g)lyph, and widgets whose "label" is really part of the value (Version's v, Git PR's PR/MR, Sandbox/Voice/Remote Control status formats) would be left out.