elrs-bind: store 20 bind phrases and scroll the history list

[raphaelhunziker1202-stack]

The bind phrase history is capped at 5 entries, and the cap is on the stored data rather than the display:

while #History.vals > History.MAX do
  table.remove(History.vals)
end

Adding a sixth phrase silently drops the oldest one from history.txt. Anyone running more than five aircraft loses phrases that are still in use and has to type them again.

Change

• New History.PAGE = 5 — the number of rows actually rendered. The on-screen footprint no longer depends on how much is stored, which is what made the low cap necessary in the first place.
• History.MAX 5 → 50 (stored entries)
• histOffset scrolls a window over the list; history_text / history_press / history_remove resolve through it
• Up/Down buttons move one page at a time, shown only when the history is longer than one page, disabled at the ends
• A position label shows 1-5 / 50
• Adding a phrase resets the offset so the new top entry is visible
• history_clampOffset() keeps the window in bounds after a delete

History.load() already reads 64 * History.MAX bytes, which covers the new limit (bind phrases are capped at 52 characters, so 50 x 53 = 2650 < 3200).

No change to the row layout, the X delete button, or any CRSF handling.

Based on the bindphrase branch since it builds on the elrs-bind tool from ExpressLRS#10.

Testing: written against the LVGL patterns already used in this repo (callback text / visible / active). Happy to adjust the nav row placement or button labels.

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