copy: rename "Donation Button" to "Donate Button" across UI and docs

[designsats]

The widget is called the Donate Button in product copy, but the repo said
"Donation Button" throughout. This renames the user-facing string everywhere it
appears, and stops at identifiers.

ab23734 (#9) made "Donation Button" consistent; this is the same sweep in the
other direction, now that the name has settled.

What changed

Commit 1 — copy across UI and docs (13 files, no behavior change)

• index.html / 404.html: <title>, page and card headings, lead copy, and the
  og:/twitter: meta tags.
• js/generator.js: the embed-snippet comments the generator emits
  (<!-- Blink Donate Button widget/script -->), kept in sync with the copy of that
  same snippet in README.md.
• README / MULTILANG-README / ANALYTICS-README, the package.json description,
  and the *-test.html pages.

Commit 2 — invoice memo + version bump (payment-path; isolated for review)

The memo was the last payer-visible "donation button" string. The two receive paths
differ, and only one is actually affected:

• Custodial: the memo is the invoice description the payer's wallet renders
  (lnInvoiceCreateOnBehalfOfRecipient and the USD variant), so payers now see
  <username> donate button.
• Self-custodial (Spark): the memo is sent as the LUD-12 comment, while the
  description comes from the LNURL metadata server-side (Payment to <username>).
  That path's payer-visible text is unchanged by this PR, as it was before it.
  blink.sv advertises commentAllowed: 255, well above the memo length, so nothing
  is truncated.

Version 1.5.1 -> 1.5.2 plus the analytics widget_version, per AGENTS.md.

Deliberately NOT renamed

Renaming these would break live embeds or split analytics history:

Identifier	Why it stays
donation-button.blink.sv (Pages path, all URLs)	every embed in the wild loads its script from this path
blink-donation-button (package.json name)	package identity
embedded_donation_button (analytics referral tag)	renaming splits the metric's history

Acceptance requirements

• Every user-facing "Donation Button" / "donation button" reads "Donate Button" /
  "donate button" — zero occurrences remain repo-wide.
• The Pages path, package name and analytics referral tag are unchanged.
• The embed snippet in README.md still matches the one js/generator.js emits.
• No public API change: BlinkPayButton.init, its config options, the container-id
  contract and the success/QR DOM behavior are untouched.
• Widget version header and analytics widget_version bumped together.
• Custodial smoke: a real small donation shows <username> donate button as the
  invoice description in the payer's wallet.

Testing specs

Automated — 147 tests pass across 11 files, eslint clean (4 pre-existing
no-unused-vars warnings, untouched):

npm ci && npm test && npm run lint

The two memo assertions move with the change:

• tests/blink-lnurl.spec.js — canonical getInvoiceFromLightningAddress
• tests/widget-spark.spec.js — the inline-copy parity test

Manual — per AGENTS.md, payment-path changes want a real smoke:

• spark-test.html against a self-custodial username. Expect the description to read
  Payment to <username> (LNURL metadata, unchanged by this PR) — the memo rides
  along as the LUD-12 comment.
• A custodial username is the case that actually changes; expect
  <username> donate button in the payer's wallet. Not yet run — needs a
  maintainer with a custodial test account.

Not addressed here: npm run format currently fails on 11 files on main (including
files this PR never touches). CI runs npm ci && npm test, so it is green. Reformatting
belongs in its own PR.

Screenshots

[pretyflaco]

Custodial smoke test: PASSED ✅

Checked out the PR head (c1d2c7e), served the widget locally, and initialized it against the custodial test account pretyflaco (BTC wallet, resolved via accountDefaultWallet). The widget took the custodial path and produced an invoice via lnInvoiceCreateOnBehalfOfRecipient:

• Invoice: lnbc10u1p4gwg2qpp539crdq2vch2hywj4wwlnm6aac4jg4v22fvf4c0yx4g6rggmdunyqdp8wpex2arevekxzcm0ypjx7mnpw3jjqcn4w36x7mscqzpuxqzuysp5xt4egkc43wk78p9l0g0jxkr74qnm7mx0wuj5uyprx4lelqmqnc9q9qxpqysgqyf2zzfcfxch7tvhw4qqpfrwnkwu9ex6cperf4akauuz7nev6cpk9muk2suy9ezxdcjc0sm55glaawavqz5ppkn7he7r2ee73zwkkc2sqjvzupv (1,000 sats, 15 min expiry)
• Decoded BOLT11 description field: pretyflaco donate button — the new memo, exactly as intended
• Rendered in Blink mobile (Send Bitcoin screen) as the note/label "pretyflaco donate button" — payer-visible string confirmed in a real wallet (screenshot below)
• Analytics URL on the widget: widget_version=1.5.2, referral=embedded_donation_button (unchanged), script path unchanged
• npm test on the PR branch: 147/147 passing across 11 files

This checks the last open acceptance item (custodial smoke). Approving.

Screenshot evidence: