docs: publish minimum and recommended system requirements - #496

The download page invited anyone on "Windows 10 & 11" with no hardware
qualifier, and nothing else -- README, install docs -- said anything about
minimum specs either. That gap is part of why #460 took as long as it did to
diagnose: reporters on a decade-old Intel iGPU had no way to know their
hardware was the likely cause before hitting a confusing recording-stop
failure.

Added a system requirements section to website/docs/installation.md (the
canonical, full version), a condensed one in README.md linking out to it, and
a short note with a link on the Windows card of the download page -- the
point where a user on old hardware would otherwise download with no warning
at all.

The Windows floor (Intel 8th Gen / Ryzen 2000, matching Camtasia's published
minimum) is chosen to sit above both hardware profiles #460 reproduced on
(Haswell/HD 4600, Skylake/HD 520) with room to spare, without reaching for an
unrealistically high bar -- checked against Intel's own driver support tiers
first, but those have moved forward enough (legacy status now reaches through
10th-11th Gen) that using them as the line would exclude hardware still in
common use. Documented only, not enforced: no startup detection or install
blocking, consistent with the project's own read that a hard block isn't
worth building without stronger signal on how many users are actually
affected.

Verified: tsc --noEmit and biome clean, docusaurus build succeeds, and a
local render of both the docs and download pages confirms the table, the
admonition, and the download-page link to #system-requirements all resolve
correctly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Three findings, all confirmed against the actual app behavior and against
each other:

- README.md's condensed Windows bullet buried the recommended tier and read
  as if "8th Gen / Ryzen 2000" were the recommended spec rather than the
  minimum, contradicting installation.md's table where 8th Gen is the floor
  and 12th Gen is recommended. Rewrote it to state both tiers explicitly,
  matching the canonical table.

- The Linux row in both files stated xdg-desktop-portal/PipeWire as an
  unconditional requirement, but the app's own capture-backend table
  (recording.md) documents a browser-capture fallback when the native helper
  is unavailable -- the requirement is native capture and system audio
  specifically, not "the app running at all." Reworded both and linked to
  the existing Platform differences section, which already describes the
  fallback's reduced capabilities.

- download.tsx's Windows card warning named only Intel generations, while
  the fuller note it links to (installation.md) also names the AMD Ryzen
  2000 series equivalent -- an AMD user on old hardware got no warning at
  all before downloading. Added the same AMD clause.

Verified: tsc --noEmit clean, docusaurus build succeeds, and a local render
of both pages confirms the reworded table cell, the #platform-differences
anchor link resolves to a real heading, and the download-page footnote now
includes AMD.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>