feat(rate-limits): add optional progress bars for 5h/7d usage

[marciogranzotto]

Summary

Adds an optional visual progress bar to the rate-limits module, matching the style the context module already uses:

5h:█░░░░░░░ 24% 🟢 (2h 15m) 7d:█████░░░ 71% 🟡 (3d 4h)

Note: GitHub's code font renders the █/░ block glyphs poorly — in a terminal the bars display correctly, identical to the context module's bar.

New options

• RATE_SHOW_PROGRESS_BAR (default: false) — render a bar between the label and the percentage. Off by default, so existing installs see no change.
• RATE_PROGRESS_BAR_WIDTH (default: empty) — per-module width override; falls back to the global PROGRESS_BAR_WIDTH.

Implementation notes

• Reuses the shared progress_bar helper from utils.sh (already covered by tests/test_utils.sh), so the bar respects PROGRESS_BAR_FILLED/PROGRESS_BAR_EMPTY and theme settings.
• Width input goes through progress_bar's existing safe_int clamping, so an untrusted project .barista.conf value can't break the arithmetic.
• Both options documented in barista.conf.

Testing

• bash tests/test_utils.sh and bash tests/test_sandbox.sh pass (27/27).
• Verified rendered output with the option on and off; with it off, output is byte-identical to current main.

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