4-27 fixes

[Distortions81]

For miners
Accepted shares should get responses a little faster, with less latency spikes.
Job and difficulty changes are handled more safely, reducing bad late-share/stale-edge behavior.
Block submissions are checked more carefully against Bitcoin Core, including edge cases where a block is already accepted.

For the status site
Hashrate and difficulty units are fixed and now scale cleanly up through EH/s and large difficulty values.
Small miner difficulties now show useful decimals instead of rounding to 0.
Hashrate confidence markers are consistent everywhere: ~ warming up, ≈ settling, no marker when stable.
Saved Workers now shows best saved-worker share and live saved-worker share rate.
Graph history, BTC price timestamps, transaction-fee updates, block-height cards, and next-difficulty-change display should update more consistently.

General polish
The UI assets are embedded in the binary now, so the dashboard is less likely to break from missing template/static files [umbrelos-gopool].
Worker/miner names render more safely in the UI.
Copy-to-clipboard works in more browser situations.
The unused experimental Stratum fast-path options were removed while keeping old config keys tolerated.

[codecov]

Codecov Report

❌ Patch coverage is 68.25397% with 100 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.06%. Comparing base (6a501e9) to head (ee0361a).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
http_fallback.go	59.21%	22 Missing and 9 partials ⚠️
status_snapshot.go	81.60%	11 Missing and 5 partials ⚠️
miner_rpc.go	40.00%	10 Missing and 2 partials ⚠️
embedded_assets.go	65.21%	4 Missing and 4 partials ⚠️
miner_rejects.go	38.46%	6 Missing and 2 partials ⚠️
miner_auth.go	65.00%	6 Missing and 1 partial ⚠️
main.go	0.00%	4 Missing ⚠️
miner_stats.go	66.66%	4 Missing ⚠️
status_display_helpers.go	60.00%	2 Missing and 2 partials ⚠️
status_server_http.go	57.14%	3 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   33.96%   33.06%   -0.91%     
==========================================
  Files         131      132       +1     
  Lines       21316    20635     -681     
==========================================
- Hits         7240     6822     -418     
+ Misses      12637    12464     -173     
+ Partials     1439     1349      -90     

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