fix(version check): remember a failed lookup and keep it off the render path

[tkuhn]

Closes #686.

What was wrong

WicketApplication.getLatestVersion() asks the unauthenticated GitHub API for the release list — 60 requests an hour per address. Once those are spent, the 403 was handled in three unhelpful ways:

• A failed lookup was not remembered. latestVersion stayed null and only success was cached, so HomePage.java:64 re-issued the request on every render — which is what kept the limit spent. Caching success forever and failure not at all is the wrong way round.
• It ran on the request thread, inline during page construction, so a slow or unreachable api.github.com was a slow home page. This is the wait PR Show outdated view results with a spinner instead of a bare spinner (#599) #600 set out to remove; the version check was missed there because it is not an API-cache query.
• It was logged as an error with a full stack trace — sixty lines of Wicket/Jetty/Maven frames for "could not reach GitHub", which reads like a broken startup when nothing is wrong.

The change

• A lookup is claimed at most once an hour (claimVersionLookup), whether it succeeds or fails; a rate-limited answer is waited out for as long as its x-ratelimit-reset says.
• Startup keeps looking the version up in the foreground — that is where the banner needs it, and nothing is being served yet. getLatestVersion() answers with whatever that left behind and hands a due lookup to the background.
• A failure is a warn with its message.
• Not knowing the version now reads as unknown in the banner and leaves the sentence unsaid on the home page, instead of "The latest public version is null."

Verification

Ran a second jetty on :37374 both ways.

Unreachable release API (constant pointed at a dead port for the run):

WARN WicketApplication - Could not fetch the latest version from http://localhost:1/releases:
    org.apache.http.conn.HttpHostConnectException: Connect to localhost:1 ... Connection refused

one line, no trace — and across three home-page renders: all 200, 1 lookup attempted in total (before this change each render would have made another). The home page said "You are running a temporary snapshot version of Nanodash (5.13.1-SNAPSHOT)." with no trailing "null".

Real API: banner Latest public version: 5.13.0, home page "…(5.13.1-SNAPSHOT). The latest public version is 5.13.0." — unchanged behaviour when GitHub answers.

Tests: 1297 pass, 6 new in LatestVersionLookupTest covering the once-an-hour claim, no lookup once the version is known, an unknown version answered with null rather than waited for, and the x-ratelimit-reset handling (honoured when later, ignored when past or unparseable).

Branched from master, independent of #687.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LqfrG3VXH5tzWXPvYJzXML

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🎉 This PR is included in version 5.14.0 🎉

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• nanodash-5.14.0
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