Adds round-trip support for ZPL's `^FC` directive that lets a single
`^FD` payload reference printer-RTC values inline. ZPL ground form:

  ^XA
  ^FO50,50^A0N,30,30^FDDate %d/%m/%Y at %H:%M^FS
  ^XZ

Prints "Date 24/05/2026 at 09:07" using the printer's clock. The
`%`, `{`, `#` chars before a token letter are the dispatch chars set
via `^FC<date>,<time>,<tertiary>` (defaults).

Stored in our model as `«clock:T»` markers in `props.content` —
same `«…»` bracket family as the `«name»` variable markers from the
^FE feature, dispatched at resolve time by the `clock:` prefix.

Three layers:

  - lib/fcTemplate.ts: clock primitives (extract/resolve/tokens-to-
    markers / markers-to-tokens / pickClockChars). Tokens supported:
    Y / y / m / d / H / M / S / j. Julian day uses month-length
    summation so DST-spring-forward days don't round down by one.
    `markersToTokens` takes just the date char (firmware treats all
    three clock chars as the same RTC). `tokensToMarkers` guards
    against empty clock chars so malformed callers don't collapse
    to a `[]` char-class that silently matches nothing.
  - parser: tracks ^FC clock chars, scans ^FD for clock-char + token
    letter sequences, converts to `«clock:T»` markers after the ^FE
    FN-embed pass. Both ^FE embedChar and ^FC clockChars reset on
    every ^XA so multi-label streams don't leak format-scoped state.
  - generator: single tree walk collects both ^FE-template and ^FC-
    clock payloads, picks safe delimiter chars, emits ^FC only when
    ≠ defaults, leaves markers literal when no safe triple exists
    (same fail-safe as ^FE).

applyBindingToObject accepts an optional `now: Date` — lazy
allocation inside the clock branch so plain-text fields don't pay
for an unused Date construction. applyBindingToTree evaluates once
per tree pass and reuses across all leaves.

Schema-mode leaves `«clock:T»` markers literal (same intent as
`«name»` placeholders: show structure, not data).

28 new unit tests (fcTemplate primitives + parser/generator
roundtrip + binding integration with fixed `now` + DST Julian-day
edge cases + multi-label state-reset).

… colour

Address gemini's PR #93 findings:

- overflow-y-hidden meant content past MAX_ROWS was unreachable
  (no scroll, no expand). Switch to overflow-y-auto with the
  scrollbar visually hidden so wheel/keyboard scrolling still works
  and the mirror layer stays pixel-aligned (a visible scrollbar
  would shift the textarea's text origin and break the highlight).
- Hardcoded `#67c8e0` clock-marker colour replaced with a themed
  --color-info CSS var. Light + dark palettes get distinct values
  so the clock highlight stays readable on both. Picker dropdown
  consumes the same token via `text-info`.